The present invention relates to a differential assembly and more particularly to a lightweight differential having a biasing preload.
Differential assemblies are well known in the art and generally include open, limited slip, and locking differentials, the simplest of which is an open differential. An open differential transfers the same amount of torque to each wheel. In low traction situations, such as driving on ice, the amount of torque applied to the wheels without one of the wheels slipping is limited to the greatest amount of torque that may be applied to the wheel with the least amount of traction. A conventional open differential is shown in FIG. 1 and includes a differential case 20, which encases and locates the side gears 41 and 42 and pinions 30. A flat thrust washer 60 is disposed between each side gear and the differential case to prevent the side gear from engaging the differential case. The thrust washers 60 are typically machined to specific tolerances to prevent preload or torque biasing.
Limited slip differentials generally include the components of an open differential as well as a clutch pack and a biasing element, such as a spring. Elements of the clutch packs rotate with the side gears and the biasing element urges the side gears apart to create an initial torque bias within the clutch pack. This initial torque bias prevents the side gears from rotating relative to each other until the initial torque bias is overcome. The magnitude of the torque bias is a function of the force applied by the biasing element, the number of frictional surfaces in the clutch pack, area of the friction surfaces, and the friction coefficient of the friction surfaces. In low traction situations, limited slip differentials create a torque bias so that the maximum amount of torque that can be applied to each wheel, without causing relative rotation of the side gears, is the torque it takes to overpower the clutch pack rather than the maximum amount of torque the wheel with the least amount of traction can sustain without slipping. The clutch pack preload typically found in limited slip differentials is greater than 108 N-m.
Four-wheel-drive or all-wheel-drive vehicles typically include a constantly driven axle 4 and a selectively engageable axle 2 (FIG. 6). Engagement of the selectively engaged axle typically includes engaging a transfer case 80 and wheel hub locks 92 (FIG. 6). When engaged, the wheel hub locks 92 ensure that the wheels rotate with their associated shafts. When the selectively engageable axle 2 is not driven, the locks 92 are disengaged to permit the wheels to rotate relative to their associated axle shafts in order to improve fuel economy. A limited slip differential may be unnecessary or undesirable on the selectively engageable axle 2. For example, including a limited slip differential on a selectively engageable axle 2, typically the front axle, increases the vehicle cost and may adversely affect steering. Therefore, it is usually desirable to use an open differential on the selectively engageable axle. One problem with using an open differential with a selectively engaged axle is that while the wheel hub locks 92 and transfer case 80 are not engaged, the output half shafts 12 and 14 may be spinning at different speeds due to inherent frictional or drag force differences between each output half shaft. If the output half shafts are spinning at different speeds, noise, vibration, and harshness may occur from the gear mesh between the side gears 40 and pinions 30 as they rotate within the differential case 20. Different speeds of the half shafts 12 and 14 may also make it difficult to engage the wheel-end hub locks and may cause engagement clash, resulting in grinding, noise, vibration, or harshness issues as the wheel-end hub locks are engaged. Therefore, it is desirable to ensure that the differential side gears 40 and output shafts 12 and 14 are rotating at the same speed.